X-ray sources of high intensity are in demand for applications ranging from medical imaging to non destructive evaluation and diffraction studies. In X-ray generating sources a cloud of electrons emitted from a cathode are accelerated to high energy and hit an anode target at a focal spot. The anode emits X-rays in response to the incident electrons. When electrons strike the anode surface only a small fraction of their energy is converted to X-rays, while the major portion of the energy is released as heat, thereby elevating the anode temperature in operation. The anodes of fixed anode X-ray sources are generally provided with a cooling fluid or air to remove the heat. Significant heat is generated at the fixed focal spot which limits the energy output or the operation time of the source.
The solution to this problem has been found in rotating the anode of the X-ray device. Rotating anode tubes have largely replaced fixed anodes; see A. Ungelenk U.S. Pat. No. 2,111,412; M. J. Gross et al, U.S. Pat. No. 2,121,630 for examples of such tubes. The use of a rotation anode spreads the heat out over a large area of the target while maintaining a narrow focal spot, and provides increased power output and longer operating times. However, even with the rotation anode design, these tubes continue to provide many problems remain such as suitable anode cooling, inadequate beating life, and problems in manufacture. The aforementioned limitations have tubes modelled after Ungelenk and Gross. In the Ungelenk tube a vacuum tight rotating seal is required to provide the rotary motion to the anode and these seals are found to leak over a period of time. The tube of Gross provides a vacuum tight envelope with the stator of an induction motor outside the vacuum enclosure and its rotor and anode mounted on bearings within the vacuum envelope. A limitation of the Gross tube is that the anode is cooled primarily by thermal radiation which is inefficient except at very high temperatures. Since the anode structure operates in a vacuum environment, the bearings supporting the motor rotor and anode cannot be lubricated after the tube is sealed, which results in a shortened bearing lifetime.
Various solutions have been proposed for correcting these disadvantages. For example, by rotating the entire vacuum enclosure, including the anode which forms part of the rotating structure, a flowing coolant can be circulated directly in back of the anode to provide efficient cooling in a vacuum tight system. Alternatively air cooling can be used by attaching cooling fins to the vacuum rotating vacuum enclosure. In these systems an electron source means must be provided within the rotating enclosure that will focus the electrons on the anode so that the X-rays will be emitted from a position fixed in space. One approach is to use proximity focusing from a hot filament or cathode that is held stationary within the rotating vacuum enclosure. Several structures have been proposed to support the cathode in stationary position within the rotating vacuum enclosure. In U.S. Pat. No. 4,878,235 Anderson uses bellows which rotate with the vacuum envelope, and provide a mechanism to fix the cathode in space by engaging mechanical bearings. In U.S. Pat. Nos. 4,788,705; 5,200,985 and 5,274,690 the cathode is supported on beatings that coincide with the axis of rotation of the vacuum enclosure. A magnetic force produced by a magnet outside of the rotating structure is used to prevent the cathode from rotating.
Although satisfactory in certain respects, the above described designs are disadvantageous in that they require an extreme precision to ensure the axis of the beatings supporting the cathode and the axis of the beatings supporting the rotating vacuum enclosures precisely coincide. Any lateral displacement of these two axes will cause the cathode to move radially as the enclosure is rotated. Any angular displacement of the two axes will cause the cathode to wobble axially so the proximity focusing of the electron beam may be disturbed. Besides, a small amount of friction in the cathode beatings can cause the cathode to wobble azimuthally, particularly when a magnetic field clamp is used. Moreover, in the X-ray tubes which use mechanical beatings within a vacuum enclosure, heat build up in the bearings is a major cause of tubes failure. The bearings are not accessible for lubrication, so that the lubrication applied during manufacture must last the life of the tube. In addition, the lubricants that can be used must have very low vapor pressure so as not to interfere with the operation of the tube. As they wear mechanical bearings often produce noise which is objectional to the personnel near the tube.